Magnetohydrodynamic generators, particularly in electrodes therefor



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F I P 85 S 2 2 Sheets-Sheet 1 Filed March 4, 11.963

March 28, 1967 z. CROITORU 3,311,762

MAGNETOHYDRODYNAMIC GENERATORS, PARTICULARLY IN ELECTRODES THEREFOR 2 Sheets-Sheet 2 Filed March 4, 1963 United States Patent O 10 Claims. (c1. s10 11 The present invention relates to magnetohydrodynamic generators, that is to say generators which directly produce electric current, without mechanical parts in movement, from a plasma passing through a channel while subjected to a magnetic field, by collection of charges, separated by this field, one negative and positive electrodes located in said channel.

The chief object of the present invention is to provide improved electrodes for magnetohydrodynamic generators, in particular concerning their aptitude to exchange electric charges with the plasma in which they are immersed, both by increasing the area of contact of such electrodes with the plasma and by giving said electrodes a shape serving to increase their capacity to inject electrons into this plasma.

The invention consists chiefly in providing such electrodes with grooves having surfaces which are nonparallel, and preferably perpendicular, to the direction of the magnetic field.

Other features of the present invention Will appear in the following description of preferred embodiments thereof, with reference to the appended drawing, given merely by way of example, and in which:

FIG. 1 diagrammatically shows, in perspective view, a portion of the channel of a magnetohydrodynamic generator;

FIG. 2 is a section on the line II-II of FIG. 1 and relates to a first embodiment of a positive electrode of the generator of FIG. 1 made according to the present invention;

FIGS. 3 and 4 are views similar to FIG. 1 and relating to two other embodiments of such an electrode, respectively.

One will first describe, with reference to FIG. 1, hw a magnetohydrodynamic generator is made.

It is known that such a generator generally comprises the following elements:

Means (for instance a heat source, not shown by the drawing) for ionizing a gas and transforming it into plasma P;

A tube, channel, or analogous conduit 1 made of a dielectric material through which the plasma flows with a speed V;

Means (preferably consisting, as shown, of an electromagnet comprising a soft iron armature 2 and an energizing coil 3) for creating in an air gap 4, an intensive magnetic field B acting in a given zone 5 of tube 1, the direction of the magnetic field being generally at right angles to the velocity V of the plasma so as to separate therein the positive charges or ions (which move in the direction S directed along vector product BX V and negative charges or electrons (which move in a direction opposed to S); and

Collecting electrodes, generally disposed against the Walls of tube 1 with their collecting surface 6a parallel to the velocity of flow of the plasma and to that of the magnetic field, to wit at least one negative electrode 7 for collecting electrons from the plasma and at least one positive electrode 8 for collecting ions from the plasma and/ or injecting electrons thereinto.

This causes the formation, between these two electrodes 7 and 8, of a potential dilference which may be used in a load circuit 9 (diagrammatically shown in the form of a resistance through which flows a current 1) and also serving to feed (once the generator has been started) coil 3, the generator thus formed transforming the energy of the heat source (not shown), which heats and ionizes the plasma, directly into electric energy that can be used in circuit 9, and this without any mechanical part in movement (only the ionized gas P and also the electric charges thereof being in movement).

It is known that, whereas the exchange of electricity between the ionized gas or plasma and the negative electrode generally involves no problem due to the fact that it takes place by absorption of electrons supplied by the gas, the exchanges of electric charges between the plasma and the positive electrode are limited for the following reasons:

On the one hand, the density of the ionic current reaching a positive electrode such as 8 is limited according to the kinetic theory of gases, to a value J depending upon the nature, the temperature and the number of ions in the plasma, and

On the other hand, the density of the electronic emission current J of electrode 8, which is limited by its temperature and its chemical composition, may be substantially reduced by the magnetic field B parallel to the surface of electrode 8. As a matter of fact, the electrons emitted in a direction F perpendicular to the surface 6 of said electrode and to the magnetic field are deflected by said field. The curving of the trajectories of the electrons may be such that they return on the emitting electrode 8.

Due to these limitations of the ionic current J coming from the ionized gas and caught by the positive electrode 8 and of the density of the electronic current J emitted by this electrode, it may happen that their sum I is smaller than the density of current 1 which could be obtained in conduit 1 if there were no limitations to the electrodes. J depends upon the intensity of the magnetic field, upon the velocity and conductivity of the plasma and upon the electrostatic field between electrodes 7 and 8. If I is lower than 1 it is not possible to provide the desired electrostatic field and the generator will work under unfavorable conditions, in particular with a reduced intensity and efficiency.

Besides, it should be pointed out that experiments have shown in a more accurate manner that, if the electromotive force of the generator is further increased, despite the limitation of current due to the phenomenons that are considered, the potential difference between the positive electrode of the generator and the ionized gas may reach a value such that discharges and even arcs are started. In this case, it is no longer possible to talk of limit current but the increase of current that is obtained in this way is ensured at the cost of a greater potential drop so that finally the efficiency of the generator is reduced.

In order to obviate these drawbacks, the electrodes, and in particular the positive electrode 8, immersed in the plasma, are provided with grooves 10, some faces or portions 6b of which are non parallel, and even preferably perpendicular to the direction of magnetic field B. Some portions 6a of the surface 6 of electrode 8 in contact with plasma P may of course remain parallel to this direction.

Owing to this arrangement of the positive electrode 8, the following results are obtained:

The area of contact between plasma P and electrode 8 is increased, being easily multiplied by two, so that there is obtained an increase of the total amount of ions that are collected and of electrons that are injected through this electrode per unit of time; it is thus possible to multiply this total amount by two;

This surface is made to comprise portions 6b which are non parallel to the magnetic field B so that the electrons emitted by these portions do not run the risk of being deflected and sent back to the electrode by the magnetic field; as the force upon an electron moving in direction F in magnetic field B is proportional to the-BXF, it

is (for a gixen value F and B) maximum when B is .9 perpendicular to F, that is to say parallel to the emitting surface (which is just the case for the Fb that correspond to portions 6a), and minimum (in fact equal to zero) when B is parallel to F, that is to say perpendicular to the emitting surface (which is just the case for the Fb which correspond to portions 6b).

Due to the presence of these portions 6b not parallel a to B, the efiiciency is further increased and it may be multiplied by a factor averaging for instance.

Finally, the ribs according to the invention have for their effect to reduce the drop of potential between the positive electrode and the ionized gas for a given value of the current that is supplied, which corresponds to an improvement in the operation and an increase of the efficiency of the generator.

The dimensions of the ribs are not critical. They advantageously range from 1 millimeter to 1 decimeter. Preferably they are equal to one or several centimeters. The widths and the depths of the ribs are generally of the same order of magnitude.

The invention provides therefore an electrode, in particular a positive electrode, which has, over the prior art electrodes, many advantages, in particular the following ones:

The number of the electrons sent back by the magnetic field is reduced in the case of an electrode that emits electrons;

The area of exchange of electricity between the electrode and the plasma is increased;

The number of electric charges exchanged per unit of time between the electrode and the plasma is finally increased;

The difference of potential between the positive electrode and the ionized gas for a given value of the current is reduced;

Finally when such an electrode is disposed in a magnetohydrodynamic generator, the intensity of the current supplied by such generator increases.

Without departing from the spirit of the invention, the geometrical arrangement disclosed by FIG. 2 might be varied to give it a slightly different form having for its effect to increase the area of the electrode in contact with the plasma and to provide it with portions of surface non parallelto the magnetic field.

FIGS. 3 and 4 show by way of non limitative examples two other shapes, to wit one with grooves of semi-circular cross section (FIG. 3) and one with grooves of semi hexagonal cross section (FIG. 4), these shapes having the advantage of a better mechanical resistance, in particular under the action of thermal shocks.

In FIGS. 3 and 4 the same reference numerals as in FIG. 2 are used for designating equivalent elements. The

ribs or grooves have surface portions 6a parallel to B which emit electrons in the directions of arrows Fa perpendicular to B and surface portions 60 oblique with respect to B, which emit electrons in the directions of arrows Fc oblique with respect to B.

The efiiciency is increased for the same reasons as in the embodiment of FIG. 2 (increase of the area of contact, reduction of the number of electrons returned to the electrode, reduction of the potential drop between the electrode and the ionized gas for a given value of the current that is supplied).

It should be pointed out that the improvements to electrodes according to the invention might also be applied in other fields (than that of magnetohydrodynamic generators) making use of an ionized gas or plasma, for instance to probes used in thermo-nuclear fusion machines.

In a general manner, While the above description discloses what are deemed to be practical and efiicient embodiments of the invention, said invention is not limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the invention as comprehended Within the scope of the appended claims.

What I claim is:

1. In a magnetohydrodynamic generator comprising an elongated channel to be traversed in operation by a longitudinal stream of ionized gas, means for providing a magnetic field across said channel substantially perpendicularly to the longitudinal direction of said stream of ionized gas in operation, at least one first electrode having a surface thereof inside said channel for collecting in operation the positive charges separated from said stream by said magnetic field, and at least one second electrode having a surface thereof inside said channel for collecting in operation the negative charges separated from said stream by said magnetic field, the improvement consisting in constituting said surface of said at least one first electrode with alternating ribs and grooves when looking in the direction of said magnetic field, said ribs and grooves having the axes thereof directed substantially parallel to the longitudinal axis of said channel.

2. The improvement according to claim 1, wherein the distance between the axes of two successive ribs is comprised between about one millimeter and about one decimeter.

3. The improvement according to claim 1, wherein said grooves and said ribs have a substantially rectangular ghtitpe in cross-section by a plane parallel to said magnetic 4. The improvement according to claim 1, wherein said grooves and said ribs have a substantially trapezoid sfih'itge in cross-section by a plane parallel to said magnetic 5. The improvement according to claim 1, wherein said grooves and said ribs have a substantially semicircular shape in cross-section by a plane parallel to said magnetic field.

6. In a magnetohydrodynamic generator comprising an elongated channel to be traversed in operation by a longitudinal stream of ionized gas, means for providing a magnetic field across said channel substantially perpendicularly to the longitudinal direction of said stream of ionized gas in operation, at least one first electrode having a surface thereof inside said channel for collecting in operation the positive charges separated from said stream by said magnetic field, and at least one second electrode having a surface thereof inside said channel for collecting in operation the negative charges separated from said stream by said magnetic field, the improvement consisting in constituting said surfaces of said at least one first electrode and at least one second electrode with alternating ribs and grooves when looking in the direction of said magnetic field, said ribs and grooves having the axes thereof directed substantially parallel to the longitudinal axis of said channel.

7. The improvement according to claim 6, wherein the distance between the axes of two successive ribs for each of said electrodes is comprised between about one millimeter and about one decimeter.

8. The improvement according to claim 6, wherein said grooves and said ribs have a substantially rectangular shape in cross-section by a plane parallel to said magnetic field.

9. The improvement according to claim 6, wherein said grooves and said ribs have a substantially trapezoid shape in cross-section by a plane parallel to said magnetic field.

10. The improvement according to claim 6, wherein said grooves and said ribs have a substantially semi-circular shape in cross section by a plane parallel to said magnetic field, i

References Cited by the Examiner OTHER REFERENCES Publication: Proceedings of Second Symposium on the Engineering Aspects of Magnetohydrodynarnics," Philadelphia, Mar. 9-10, 1961; edited by Manual and Mather; Columbia University Press, New York and London; pp. 25-27, 59 and 60.

MILTON O. HIRSHFIELD, Primary Examiner, DAVID X. SLINEY, Examiner, 

1. IN A MAGNETOHYDRODYNAMIC GENERATOR COMPRISING AN ELONGATED CHANNEL TO BE TRAVERSED IN OPERATION BY A LONGITUDINAL STREAM OF IONIZED GAS, MEANS FOR PROVIDING A MAGNETIC FIELD ACROSS SAID CHANNEL SUBSTANTIALLY PERPENDICULARLY TO THE LONGITUDINAL DIRECTION OF SAID STREAM OF IONIZED GAS IN OPERATION, AT LEAST ONE FIRST ELECTRODE HAVING A SURFACE THEREOF INSIDE SAID CHANNEL FOR COLLECTING IN OPERATION THE POSITIVE CHARGES SEPARATED FROM SAID STREAM BY SAID MAGNETIC FIELD, AND AT LEAST ONE SECOND ELECTRODE HAVING A SURFACE THEREOF INSIDE SAID CHANNEL FOR COLLECTING IN OPERATION THE NEGATIVE CHARGES SEPARATED FROM SAID STREAM BY SAID MAGNETIC FIELD, THE IMPROVEMENT CONSISTING IN CONSTITUTING SAID SURFACE OF SAID AT LEAST ONE FIRST ELECTRODE WITH ALTERNATING RIBS AND GROOVES WHEN LOOKING IN THE DIRECTION OF SAID MAGNETIC FIELD, SAID RIBS AND GROOVES HAVING THE AXES THEREOF DIRECTED SUBSTANTIALLY PARALLEL TO THE LONGITUDINAL AXIS OF SAID CHANNEL. 